A) Field of the Invention
The present invention relates to a method and apparatus to be used for inspecting stains.
B) Description of the Related Art
One of defects formed in a charge coupled device (CCD) solid state image pickup device, a MOS solid state image pickup device, a liquid crystal display (LCD) and a plasma display panel (PDP) is a luminance change like a “stain” appearing when a uniform white image is photographed or displayed. The “stain” includes a stain brighter than a peripheral area and a stain darker than a peripheral area, respectively called a “white stain” and a “black stain”. Namely, the “white stain” has a luminance of the inner area of the stain higher than that of the peripheral area of the stain, whereas the “black stain” has a luminance of the inner area of the stain lower than that of the peripheral area of the stain. “Irregularity” indicates the state that a luminance change extends in a broad area.
The “stain” means “components changing relatively smoothly and locally in output signal unevenness in a bright state, and a plane defect in a relatively large area of a screen to be caused by dirt of a lid glass or photodiodes”. This definition is applied flexibly. For example, “irregularity” is assumed to be included in “stain”.
For example, a photographed image of a color solid state image pickup device has in some cases a very low luminance irregularity (white stain, black stain and the like) to be caused by defects formed during a process of manufacturing color filters of three primary colors: red (R), green (G) and blue (B).
A brightness of an image on a solid state image pickup device or a display device is not even over the whole area, but the central image area is bright and the peripheral image area is dark. This characteristic is called luminance shading.
Almost all images taken with a solid state image pickup device have luminance shading. A difference between a highest luminance (luminance of a central image area) value and a lowest luminance (luminance of an image peripheral area) value is generally about several % to 10% of an average value of a luminance of the whole image.
The “stain” is also a defect caused by luminance unevenness. However, a stain luminance and a stain peripheral area luminance have often a difference of only several %, and this difference has a level very near to a noise level. A luminance level difference from a luminance of a peripheral image area is small and the luminance at a boundary (edge portion) between the stain portion and the other portion is approximately the same level as that of the peripheral image area, without a steep luminance change. In such a state, it is difficult to detect a stain by a method of simply obtaining a luminance change (slope), because the influence of luminance shading exists additionally.
The present circumstance is that automatic stain inspection is difficult and inspection is made visually by inspectors (for example, refer to JP-A-HEI-2003-329597). This is because human eyes can recognize often a luminance difference clearly even if a difference between measured luminance values is not large.
However, since visual inspection by human eyes is subjective evaluation, a variation in inspection results by inspectors is likely to occur. Another problem is that time and cost required for inspections become enormous. Automatic inspection has therefore been studied.
A method has been disclosed for extracting an edge by using a spatial filter utilizing a first order difference of the Prewitt or Sobel operator of 3×3 pixels (for example, refer to JP-A-HEI-10-206344).
However, if an image has the luminance shading which essentially has an inclination, it is often difficult to detect an edge by using a Sobel filter.
Techniques of detecting a stain have been disclosed excluding the influence of luminance shading (for example, refer to JP-A-HEI-9-329527)
First, noise components are eliminated from an original image by using a low-pass filter (LPF). An image having only the luminance sharing characteristic is therefore obtained. Next, the obtained luminance characteristic is subtracted from the original image. In this manner, the luminance shading is excluded from the image to detect stains.
This method is effective for detecting, for example, flaws. However, since LPF is used, a difference between luminances and a change (inclination) amount in luminances at an edge become smaller so that stains are difficult to be detected.
Further, it is necessary to prepare separately a means for eliminating luminance shading and a means for detecting an edge. Furthermore, it takes a time to detect a stain.
Techniques of removing luminance shading have been disclosed using a planarizing module with a spline function (for example, refer to JP-A-HEI-11-66311).